A bearing is a machine element that is used in many machines and apparatuses ranging from daily goods to industrial products and is used in a broad range of environments from room temperature to high temperature. For example, the bearing is used under high-temperature environments in the field of manufacturing including semiconductors and liquid crystal panels and in the field of sophisticated industry including thermal treatment equipment.
However, the limit of its practical working temperature is approximately 300° C. because of material constraints. For example, a bearing formed of martensite stainless steel, heat resisting steel for bearings or the like is abruptly reduced in hardness with rise of the working temperature, and therefore the limit of the working temperature is approximately 300 to 400° C. considering practical life. Accordingly, when a bearing is used under a high-temperature environment, a cooling device for cooling the bearing is provided or the bearing is isolated from a high-temperature environment (for example, inside of a furnace).
From such a background, development of a heat-resistant bearing usable under high-temperature environments has been desired. As a bearing that can have a long life even under a high-temperature and special environment, for example, there has been known a bearing in which a base material of rolling elements is bearing steel or stainless steel and surfaces of the rolling elements are nitrided (see Patent Document 1, for example).